\begin{tikzpicture}
    \tkzDefPoints{0/0/O}
    \tkzDefPoint(0:1.5){A}
    \tkzDefPoint(40:1.5){B}

    \tkzDrawCircle[thick](O,A)
    \tkzDrawSegments(O,A  O,B)
    \tkzMarkAngle[size=.6, -latex](A,O,B)
    \tkzLabelAngle[pos=.8](A,O,B){$\alpha$}
    \tkzDrawPoint(O)
    \tkzLabelPoints[left](O)
    \tkzLabelPoints[right](A,B)
\end{tikzpicture}

